1. Field of the Invention
The present invention relates to an anti-vibration device having a variable-apex prism and, in particular, to an anti-vibration device having a variable-apex prism for use in optical apparatuses (such as video or still cameras) and capable of correcting image blur due to vibrations, etc. by arbitrarily changing the propagation direction of a transmitted light flux.
2. Description of the Related Art
Recently, progress has been made in automatization of photographing operations with camera apparatuses, such as still and video cameras; and various means for realizing automatized functions, for example, automatic exposure control means and automatic focusing means, have been put into practical use.
As one of such means, a blur correction means for mitigating undesirable blur in a picture (a so-called image blur) due to various causes has been devised and is being put into practical use.
In particular, in a camera apparatus like a video camera, zoom lenses are generally used as the photographing lens, and the zoom ratio of such lenses has been increasing year-by-year.
Apart from this, a marked reduction in camera size has been achieved. As a result of the reduction in the size of the imaging plane, progress in the high-density mounting technique, the development of a small-sized recorder-mechanism chassis, etc., there has even appeared a small-sized camera apparatus which allows photographing with one hand.
However, such a small-sized video camera with a zoom lens has a problem in that it is subject to the generation of an undesirable image blur attributable to camera-shake during photographing.
Various anti-blurring means have been proposed in order to eliminate such a blur and obtain a stable image. Such anti-blurring means prove greatly effective not only in eliminating undesirable blur due to a shake of a hand-held camera but also in trying to mitigate an image blur in a situation in which camera-shake cannot be avoided even with a tripod, as in the case of photographing in a ship or an automobile.
Such an anti-blurring means includes at least a camera-movement detecting means for detecting a camera movement, and a blur correction means for compensating for any camera movement in accordance with the detected camera-movement information so as to prevent an image blur from appearing in the obtained image.
Known examples of the camera-movement detecting means include an angular acceleration meter, angular speed meter and angular displacement meter. As an example of the blur correction means, a method using a variable-apex prism is available. Further, in a video camera of the type in which an image area to be actually used as a picture plane is extracted from the entire image information obtained, a blur correction method can be used according to which the image-area extracting position is sequentially corrected in such a way as to compensate for any undesirable camera movement.
A blur correction means of the former type, in which any blur is removed by an optical means like a variable-apex prism at the stage in which the image is formed on the imaging device, will be referred to as an optical correction means, and a blur correction means of the latter type, in which blur is removed, for example, by electronically changing the extracting position of the image information containing the blur, will be referred to as an electronic correction means.
Generally speaking, an optical correction means is capable of correcting any blur within a fixed angle, determined as the blur angle, of a camera irrespective of the focal distance of the photographing lens. Accordingly, it can exhibit a blur eliminating property acceptable for practical use even when the telescope-end focal distance of the zoom lens is relatively long.
In an electronic correction means, in contrast, the correcting ratio with respect, for example, to the vertical dimension of the picture plane, is constant. Accordingly, the blur eliminating effect deteriorates in proportion to the telescope-end focal distance of the zoom lens.
Next, an anti-blurring apparatus having a variable-apex prism will be described.
FIG. 6(A) illustrates the relationship between the focal distance of the photographic lens and the angle of movement of a camera in terms of the position of the subject in an image.
In the drawing, numeral 23 indicates the optical axis of the photographic lens of a camera CA when the camera is in a position as indicated by solid line 22. In this position, the camera substantially catches the central portion of the face of a person 21 constituting the subject. Assuming that the camera CA has moved (as a result of camera-shake) to a position as indicated by a two-dot chain line 24, the optical axis of the camera becomes that indicated by numeral 25.
FIGS. 6(B) and 6(C) show images and picture plane positions obtained when the camera CA is in positions 22 and 24, respectively. FIG. 6(B) shows the condition when the zoom lens is at the telescopic end, and FIG. 6(C) shows the condition when it is at the wide angle. Numeral 26 indicates the subject inside the picture plane; numerals 27 and 29 indicate the picture plane positions when the camera CA is in the position 22; and numerals 28 and 30 indicate the picture plane positions when the camera CA is in the position 24.
As is apparent from FIGS. 6(A) to 6(C), if the angle of camera movement a is the same, the resulting blur in the picture plane naturally becomes more undesirable in proportion to the focal distance of the photographic lens. Thus, an optical means using a variable-apex prism is especially effective when applied to a blur removing means to be combined with a photographic lens having a long telescopic focal length.
FIGS. 7(A) through 7(C) show the construction of a variable-apex prism. In the drawing, numerals 31 and 33 indicate transparent glass plates; and numeral 37 indicates a bellows section made of a material like polyethylene. A transparent liquid such as silicon oil is sealed in the space defined by the glass plates 31 and 33 and the bellows section 37.
In the condition shown in FIG. 7(B) , the two glass plates 31 and 33 are parallel to each other, and the incident and outgoing angles of a light beam 35 incident on this variable-apex prism are equal to each other.
When, as shown in FIGS. 7(A) and 7(C), the variable-apex prism exhibits some angle, the light beam 35 is bent at an angle. Thus, when the camera is inclined or undergoes a vibration as a result of camera-shake or the like, the angle of the variable-apex prism (the apex angle) is controlled in such a way that the light beam transmitted therethrough is bent in correspondence with that inclination, etc., thereby removing any undesirable blur.
FIGS. 8(A) and 8(B) show the way such blur removal is effected. In the condition shown in FIG. 8(A), the variable-apex prism is in a parallel position and the head of the subject is on the optical axis of the camera. If, as shown in FIG. 8(B), a camera movement occurs at an angle of a, the variable-apex prism is driven so as to bend the light beam, with the result that the photographing optical axis of the camera remains the same, and the head of the subject continues to be on the optical axis.
FIG. 9 schematically shows an example of an anti-vibration device which includes a variable-apex prism as described above, an actuator section for driving the prism, and an apex angle sensor for detecting the angular condition of the camera.
Since an actual image blur can occur in any direction, the front and rear glass planes of the variable-apex prism are rotatable on axes which are 90.degree. deviated from each other, as shown in FIG. 9. The components belonging to the front and rear glass planes of the prism will be indicated by suffixes a and b, respectively, attached to the reference numerals, the components indicated by the same numerals having the same functions. For clarity, some of the components to be indicated by numerals with the suffix b are not shown.
Numerals 31 and 33 indicate glass plates, and numeral 37 indicates a bellows section made of polyethylene or the like.
Numeral 51 indicates a VAP (variable-apex prism), which includes the glass plates 31 and 33, the bellows section 37, etc. A transparent liquid such as silicon oil is sealed in the inner space defined by the glass plates 31 and 33 and the bellows section 37.
Numeral 38 (38a, 38b) indicates a frame to which the glass plate 31, 33 is integrally joined by adhesive or the like.
The frame 38 forms a rotation axis 43a, 43b together with a stationary member (not shown) and is rotatable on this axis. The dimension of the rotation axis 43a is 90.degree. deviated from the dimension of the rotation axis 43b. A coil 45 (45b is not shown) is integrally provided on the frame 38, and magnets 46 and yokes 47 and 48 are provided in a stationary section (not shown).
By passing electric current through the coil 45, the glass plate 31, 33 of the variable-apex prism rotates around the axis 43. A slit 39 (39b is not shown) is provided at the tip of an arm 40 (40b is not shown) integrally extending from the frame 38, and constitutes an apex angle sensor between a light emitting device 41 (41b is not shown) like an iRED, and a light receiving device 52 (52b is not shown) like a PSD, which devices are provided in stationary sections.
FIG. 10 is a block diagram showing an anti-vibration lens system in which an anti-vibration means having the above-described variable-apex prism serving as an anti-blurring means is combined with a photographic lens.
In the drawing, numeral 51 indicates a variable-apex prism; numerals 53 and 54 indicate apex angle sensors; numeral 62 indicates an image sensor; numerals 63 and 64 indicate detection circuit sections for amplifying the outputs of the apex angle sensors 53 and 54; numeral 55 indicates a microcomputer; and numerals 56 and 57 indicate camera-movement detection means. The microcomputer 55 decides on the electric current to be supplied to actuators 58 and 59 so as to control the angle of the variable-apex prism 51 on the basis of the angle information detected by the apex angle sensors 53 and 54 and the detection results obtained by the camera-movement detection means 56 and 57, thereby setting the prism 51 at an optimum angle to remove any blur.
In the example Shown, the principal components are built in two blocks on the assumption that the glass plates 31 and 33, the rotation axes of which are 90.degree. deviated from each other, are individually controlled.
When the variable-apex prism is used as a correction means, the following equation holds true within a range in which the apex angle is relatively small: EQU .epsilon.=(n-1).sigma. (1)
where n is the refractive index of the variable-apex prism; .sigma. is the prism apex angle; and .epsilon. is the angle (the correction angle) between the incident and outgoing angles of the light beam. For example, when n=1.4, inclining the variable-apex prism by 5.degree. results in the light beam being bent by 2.degree..
In the above conventional example, blur correction is effected by independently rotating the two glass plates. When, for example, the rotation axes of the two glass plates are horizontal and vertical, respectively, and the same maximum rotation angle is imparted to them, the maximum correction amount obtained along a dimension extending obliquely on the picture plane is larger than the maximum correction amount obtained along the horizontal or vertical dimension.
Thus, it has been necessary to set the variable-apex prism at a large angle beforehand, taking this difference in maximum correction angle into consideration.
Here, the difference in maximum correction angle between the dimension of a rotation axis and a dimension different from that will be explained with reference to FIGS. 2(A) and 2(B) and FIGS. 3(A) and 3(B).
In the drawings, numeral 31 indicates the front glass plate of a (VAP) variable-apex prism 1 rotatable around a rotation axis Y, and numeral 33 indicates the rear glass plate thereof rotatable around a rotation axis Z.
Suppose the glass plate 33 has been rotated in the yaw direction by an angle .theta.y in order to compensate for a camera movement occurring along a dimension at an angle O with respect to the rotation axis Y, as shown in FIG. 2(A).
As shown in FIG. 2(B), the section taken at the angle O and passing the center of the VAP 1 has a V-shaped configuration, and the apex angle of this section will be indicated by .theta.'.
Further, suppose, as shown in FIG. 2(A), the glass plate 31 has been inclined by .theta.p around the Y-axis. Assuming that the inclination of this glass plate at the angle O is .theta..sub.1 ', then as shown in FIG. 3(A), EQU tan .theta..sub.1 '=sin O.multidot.tan .theta.p (2)
Similarly, when, as shown in FIG. 2(B), the glass plate 33 has been inclined by .theta.y around the axis Z, then as shown in FIG. 3(B), EQU tan .theta..sub.2 '=cos O.multidot.tan .theta.y (3)
Accordingly, in the case of FIG. 2, the angle .theta.' can be obtained from the following formula: ##EQU1## Thus, .theta. is larger than .theta.p or .theta.y.
When the maximum blur correction angle required by the VAP1 is defined as .epsilon.max, this maximum correction angle is in the following relationship with the rotation angles .theta.pmax and .theta.ymax, as shown in equation (1): EQU .epsilon.max=(n-1).multidot..theta.pmax (5) EQU .epsilon.max=(n-1).multidot..theta.ymax (6)
Accordingly, the rotation angles .theta.y and .theta.p are to be controlled within the following ranges: EQU -.theta.yamx.ltoreq..theta.y.ltoreq..theta.ymax (7) EQU -.theta.pamx.ltoreq..theta.p.ltoreq..theta.pmax (8)
However, as stated above, the prism apex angle .theta. when compensating for a camera movement occurring along a dimension different from those of the rotation axes is larger than the rotation angle .theta.p or .theta.y, so that the prism apex angle .theta.max' when both the rotation angles around the rotation axes are the maximum rotation angles .theta.pmax and .theta.ymax (which is the case when correcting, as much as possible, a blur occurring along a dimension obliquely extending on the picture plane), is larger than the rotation angle .theta.pmax or .theta.ymax. Assuming that the maximum blur correction amount at this time is .epsilon.max', EQU .epsilon.max'=(n-1).multidot..theta.max' (9)
Accordingly, the maximum blur correction angle .epsilon.max' obtained along a dimension diagonally extending on the picture plane is larger than the initially determined maximum blur correction angle .epsilon.max along the horizontal/vertical picture dimensions.
Thus, the above conventional device has a problem in that the diameter of the variable-apex prism usually has to be large enough to be the size required when correcting a blur occurring along a dimension extending diagonally on the picture plane, with the result that it is a diameter larger than the one required in the blur correction along rotation axes.
Further, transverse chromatic aberration due to the variable-apex prism also increases in proportion to the blur angle, so that chromatic aberration in the periphery of the picture plane increases when a blur occurring along a direction extending diagonally on the picture plane is corrected.